I built my last road bike with 42 cm width bars, but after a lot of miles they didn't really feel right when out of the saddle climbing. On my MTB the bar ends are pretty wide-spaced and have a good strong and stable feel climbing. I switched to 44 cm bars on the road bike, and its a much better feel - a good compromise between aerodynamics and leverage.
All the stuff about breathing easier is nonsense, and I don't think the standard formula is worth much - its how you ride and how things work for you.